Conventional articles of athletic footwear have included two primary elements, namely an upper and a sole member or structure. The upper provides a covering for the foot that securely receives and positions the foot with respect to the sole structure. In addition, the upper may have a configuration that protects the foot and provides ventilation, thereby cooling the foot and removing perspiration. The sole structure generally is secured to a lower portion of the upper and generally is positioned between the foot and the ground. In addition to attenuating ground reaction forces (i.e., imparting cushioning), the sole structure may provide traction and control foot motions, such as pronation. Accordingly, the upper and the sole structure operate cooperatively to provide a comfortable structure that is suited for a variety of ambulatory activities, such as walking and running.
The sole member or structure of athletic footwear generally exhibits a layered configuration that includes a comfort-enhancing insole, a resilient midsole formed from a polymer foam material, and a ground-contacting outsole that provides both abrasion-resistance and traction. The midsole is the primary sole structure element that attenuates ground reaction forces and controls foot motions. Suitable polymer foam materials for the midsole include ethylvinylacetate or polyurethane that compress resiliently under an applied load to attenuate ground reaction forces. Conventional polymer foam materials are resiliently compressible, in part, due to the inclusion of a plurality of open or closed cells that define an inner volume substantially displaced by gas.
As noted above, various impact attenuating elements and systems have been known, including such elements and systems for use in footwear products. Conventionally, the insole, midsole, and/or outsole portions of footwear products may include foam or other materials that attenuate shock and dampen vibrations, e.g., in the heel and/or toe areas of a shoe. In at least some instances, a relatively large volume of foam or other material may be needed to fully or sufficiently attenuate the impact force to which footwear products are subjected and to provide sufficient support and/or comfort. This is particularly true for athletic footwear, which may be subjected to relatively high impact forces, e.g., from running, jumping, twisting, changing directions, participating in athletic field events, and the like. An excessively high volume of impact attenuating material, if necessary to adequately attenuate ground reaction forces and/or provide support, may make the shoe stand too tall vertically, particularly for use as an athletic shoe.
Moreover, even when conventional foam materials provide adequate impact attenuation and comfort properties for use in footwear products, these materials do little or nothing in returning energy back to the footwear user. Rather, foam materials typically recover from compression and return to their original shape relatively slowly and with little or no return or “spring-back”. Additionally, if a compression force persists on the foam material, this force may further prevent or slow the material's recovery.
Accordingly, it would be useful to provide effective impact attenuating devices that attenuate impact forces (e.g., ground reaction forces), and optionally provide return or “spring-back” energy, e.g., for use in footwear products and/or other foot-receiving devices. Advantageously, such impact attenuating devices will provide these useful properties without excessively adding to the height of the footwear or other product.